Antibiotic assay

Last updated: Wednesday, 12, September, 2007

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Item Process

At least 1 mL blood in plain tube.



Reference Interval

Depends on antibiotic and dose regimen.


Monitoring plasma levels of potentially toxic drugs such as aminoglycosides, vancomycin, flucytosine and, in neonates, chloramphenicol.


For once-daily dosage of gentamicin, tobramycin or amikacin: a single measurement of plasma concentration should be made 6-14 hours after a dose.

Interpretation of aminoglycoside levels for monitoring once-daily dosage.

If the measured concentration of gentamicin or tobramycin, or one quarter the measured concentration of amikacin, does not lie between the curves on the graph then dose adjustment is required.

For 8- and 12- hourly dosage of aminoglycosides, a peak level is measured to assess adequacy of the dose in serious infections.

The peak level should be measured 30 minutes to 1 hour after a dose, given either as an IV infusion over 20-30 minutes or as an IM injection.

A trough level (immediately prior to the next dose) is measured to assess whether accumulation is occurring.

The following are recommended levels for 8-12 hourly Aminoglycoside Therapy:
Gentamicin: >8 mg/L (peak), <2 mg/L (trough)
Tobramycin: >8 mg/L (peak), <2 mg/L (trough)
Amikacin: >32 mg/L (peak), <8 mg/L (trough) 

For vancomycin, peak levels do not correlate with either efficacy or toxicity. Trough levels should be maintained at 10-20mg/L.

Flucytosine toxicity occurs at levels >100 mg/L. Trough levels should be maintained at >25 mg/L for efficacy.


Victorian Drug Usage Advisory Committee. Antibiotic Guidelines. 13th ed. 2006